@angular-builders/custom-webpack with @angular-devkit/build-angular version 0.1000.0-next.0 throws error - ERROR in Cannot read property 'flags' of undefined

Describe the Bug

This issue is based on the issue https://github.com/angular/angular-cli/issues/17499

When using @angular-builders/custom-webpack with @angular-devkit/build-angular version 0.1000.0-next.0 throws error - ERROR in Cannot read property 'flags' of undefined

Minimal Reproduction

More details on the issue can be found at https://github.com/angular/angular-cli/issues/17499

The issue and possible solution is further explained in comment https://github.com/angular/angular-cli/issues/17499#issuecomment-617303792

Expected Behavior

It should compile successfully without throwing error.

Environment

Libs
- @angular/core version: 10.0.0-next.2
- @angular-devkit/build-angular version: 0.901.0
- @angular-builders/custom-webpack: 9.1.0-beta.0

Angular CLI: 10.0.0-next.0
Node: 13.13.0
OS: win32 x64

Angular: 10.0.0-next.2
... animations, common, compiler, compiler-cli, core, forms
... language-service, platform-browser, platform-browser-dynamic
... platform-server, router, service-worker
Ivy Workspace: Yes

Package                                    Version
--------------------------------------------------------------------
@angular-devkit/architect                  0.900.7
@angular-devkit/build-angular              0.901.0
@angular-devkit/build-optimizer            0.900.7
@angular-devkit/build-webpack              0.900.7
@angular-devkit/core                       10.0.0-next.0
@angular-devkit/schematics                 10.0.0-next.0
@angular/cdk                               9.2.1
@angular/cli                               10.0.0-next.0
@angular/fire                              6.0.0-rc.1
@angular/google-maps                       9.2.1
@angular/material                          9.2.1
@angular/pwa                               0.1000.0-next.0
@ngtools/webpack                           9.0.7
@nguniversal/module-map-ngfactory-loader   9.0.0-next.9
@schematics/angular                        10.0.0-next.0
@schematics/update                         0.1000.0-next.0
rxjs                                       6.5.5
typescript                                 3.8.3
webpack                                    4.41.2

Additional Context

Add any other context about the problem here.

[just-jeb]

angular-builders 9 do not support Angular 10. Use 10.0.0-beta.0.

[seangwright]

I’m currently seeing this with consistent v9.1.x Angular dependencies.

package.json

 "dependencies": {
    "@angular/animations": "9.1.3",
    "@angular/cdk": "9.2.1",
    "@angular/common": "9.1.3",
    "@angular/compiler": "9.1.3",
    "@angular/core": "9.1.3",
    "@angular/forms": "9.1.3",
    "@angular/platform-browser": "9.1.3",
    "@angular/platform-browser-dynamic": "9.1.3",
    "@angular/router": "9.1.3",
    // ...
  },
  "devDependencies": {
    "@angular-builders/custom-webpack": "9.1.0",
    "@angular-devkit/build-angular": "0.901.3",
    "@angular/cli": "9.1.3",
    "@angular/compiler-cli": "9.1.3",
    "@angular/language-service": "9.1.3",
    "ts-node": "8.9.0",
    "tslint": "6.1.1",
    "typescript": "3.8.3",
    // ...
}

angular.json

"architect": {
  "build": {
    "builder": "@angular-builders/custom-webpack:browser",
    "options": {
      "customWebpackConfig": {
        "path": "./custom-webpack.config.js",
        "mergeStrategies": {
          "optimization.minimize": "replace"
        }
      },

custom-webpack.config.js

const TerserPlugin = require('terser-webpack-plugin');

module.exports = {
    optimization: {
        minimizer: [
            new TerserPlugin({
                parallel: 4,
                sourceMap: true,
            }),
        ],
    },
};

Running ng build "-c" "production" results in ERROR in Cannot read property 'flags' of undefined.